Female Reproductive System PDF

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 9: Reproductive System 328 Lesson 9.3 **Female Reproductive System** Introduction The female reproductive system participates in human reproduction by generating haploid (1*n*) gametes known as **oocytes** and provid...

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 9: Reproductive System 328 Lesson 9.3 **Female Reproductive System** Introduction The female reproductive system participates in human reproduction by generating haploid (1*n*) gametes known as **oocytes** and providing a supportive environment for fertilization and growth of offspring. The concepts in this lesson provide an overview of female reproductive anatomy, the process of gamete formation via **oogenesis**, and hormonal regulation of the female reproductive system. 9.3.01 Female Reproductive Anatomy The internal and external structures of the female reproductive system are responsible for producing gametes (ie, oocytes), receiving sperm, and providing a supportive environment for fertilization, gestation, and nourishment of offspring. As depicted in Figure 9.10, the following structures are associated with the female reproductive system: **Ovaries** are reproductive gonads that secrete sex hormones (eg, estrogens, progesterone) and serve as the sites of oogenesis. The **uterine (fallopian) tubes** are a pair of muscular tubes originating from the uterus and extending laterally toward each ovary. Although the open ends of the uterine tubes are near the ovaries, there is not a direct connection between the ovary and uterine tube. The open ends of uterine tubes are surrounded by **fimbriae**, fingerlike projections that facilitate direction of the oocyte from the abdominal cavity into the uterine tubes. Ciliated cells line the interior of the uterine tubes and help propel the oocyte toward the uterus. Fertilization typically takes place in the uterine tubes. The **uterus** is a muscular organ responsible for protecting and nourishing the embryo and fetus. The inner lining of the uterus, called the **endometrium**, undergoes cyclical changes in thickness at different points in the menstrual cycle. The uterus also contains a thick layer of smooth muscle called the **myometrium**, which contracts during menstruation and childbirth. The **cervix** is the most inferior portion of the uterus and serves as the opening into the vagina. The **vagina** is a muscular tube that functions in elimination of menstrual fluids during the menstrual cycle, in reception of the penis during sexual intercourse, and as the final segment of the birth canal during childbirth. **Female external genitalia (vulva)** include the folds of the **labia majora** (singular: **labium majus**) and **labia minora** (singular: **labium minus**), which protect the external opening of the vagina and the **clitoris**. The external portion of the clitoris lies at the junction of the labia minora folds. Stimulation of the densely innervated clitoris triggers genital changes (eg, lubrication, pH changes) that facilitate reproductive success. **Mammary glands** are accessory glands located within the chest wall that become fully developed during pregnancy to facilitate **lactation** (ie, the synthesis and secretion of breast milk) and nursing. Chapter 9: Reproductive System 329 **Figure 9.10** Structures of the female reproductive system. The ovaries are a pair of female gonads homologous (ie, possess the same embryonic origin) to testes. Each ovary is surrounded by an outer fibrous capsule covered by epithelial cells. Within an ovary, the cortex contains many **ovarian follicles** in various stages of maturation. Each follicle contains an immature primary oocyte surrounded by supporting epithelial cells (Figure 9.11). In the early stages of follicle maturation, the supporting follicular cells proliferate and mature into **granulosa cells**. Granulosa cells support developing oocytes by secreting sex steroid hormones such as estrogens (eg, estradiol) during the ovarian cycle. A diagram of a person\'s uterus Description automatically generated Chapter 9: Reproductive System 330 **Figure 9.11** Ovary structure. 9.3.02 Oogenesis **Oogenesis** begins during embryogenesis, as **oogonia** (stem cells) within developing ovaries undergo continuous rounds of [mitotic division](javascript:void(0)) to yield identical diploid (2*n*) daughter cells. Unlike spermatogonia, which initiate [meiosis I](javascript:void(0)) beginning at puberty, oogonia initiate meiosis I to form **primary oocytes** during the fetal period. Primary oocytes are arrested (ie, paused) in late prophase I and do not resume the stages of meiosis I again until the beginning of puberty, in response to hormonal changes (see Concept 9.3.03). At the onset of puberty, these primary oocytes are surrounded by follicular support cells in the ovary, forming **ovarian follicles**. Hormonal changes during each menstrual cycle typically result in a single ovarian follicle being selected to continue meiosis and proceed with **ovulation**. At the completion of meiosis I, unequal cell division of the chosen primary oocyte results in cells of two different sizes: a larger haploid (1*n*) **secondary oocyte** and a smaller haploid cell called the **first polar body** (Figure 9.12). In humans, the first polar body typically undergoes [apoptosis](javascript:void(0)) and degenerates but may complete meiosis in some cases. The secondary oocyte initiates [meiosis II](javascript:void(0)) but arrests at metaphase II until fertilization.  Chapter 9: Reproductive System 331 **Figure 9.12** Oogenesis. A diagram of a diagram of a person\'s life cycle Description automatically generated Chapter 9: Reproductive System 332 During ovulation, the ovarian follicle ruptures and the secondary oocyte, surrounded by a thick glycoprotein matrix known as the **zona pellucida** and a group of granulosa cells known as the **corona radiata**, is released into the abdominal cavity. The secondary oocyte is drawn into a **uterine (fallopian) tube**, where fertilization by a sperm cell can occur. In the event of successful fertilization, the secondary oocyte then completes meiosis II to form one large **ovum** (ie, fully mature gamete) and a small **second polar body** that degenerates (Figure 9.12). After fertilization is complete, the male and female **pronuclei** are fused (see Concept 10.1.01) to form a diploid **zygote**. During oogenesis, each oogonium yields one haploid gamete (ovum) and two to three haploid polar bodies, as opposed to the four haploid gametes (sperm) generated during spermatogenesis. Oogenesis begins before birth and can result in mature gametes 13--50 years later. As levels of sex hormones (eg, estrogens, progesterone) decline with age during a state known as **menopause**, oogenesis ceases. Unlike spermatogenesis, which typically occurs throughout the lifespan after the onset of puberty, it is thought that a finite number of primary oocytes are formed during the fetal period that are not typically replenished later in life. 9.3.03 Hormonal Control of the Female Reproductive System During the early weeks of embryogenesis, an XY embryo typically begins to develop male reproductive organs initiated by *SRY* gene expression (see Concept 9.2.03). In XX embryos, *SRY* is absent and development of testes typically does not occur. As a result, **anti-Müllerian hormone (AMH)**, is not produced and the development of female reproductive structures derived from **Müllerian (paramesonephric) ducts** is promoted. Because testes are not formed, **Wolffian (mesonephric) ducts** degenerate due to a lack of testosterone. The first stage of oogenesis begins during the fetal period with the production of follicles containing primary oocytes, as discussed in Concept 9.3.02. During infancy and childhood, sex hormone levels are low and progression of oogenesis is repressed; therefore, sexual development remains dormant until the onset of puberty. At puberty, the secretion of **estrogens** and **progesterone** (ie, sex steroid hormones) promotes the growth and maturation of female reproductive organs, development of secondary sex characteristics, resumption of oogenesis, and initiation of the menstrual cycle. The **hypothalamic-pituitary-gonadal (HPG) axis** regulates a series of cyclical changes that make reproduction possible (Figure 9.13). The female reproductive cycle includes the **ovarian cycle** (ie, events that occur during the maturation of a primary oocyte) and the **uterine (menstrual) cycle** (ie, events that occur in the uterus to prepare for pregnancy). The ovarian and uterine cycles occur concurrently, with a single female reproductive cycle lasting an average of 28 days. Synchronization of the ovarian and uterine cycles provides optimal conditions for the support of fertilization and early pregnancy. Chapter 9: Reproductive System 333 **Figure 9.13** Regulation of the female reproductive system. The ovarian cycle can be divided into three phases (Figure 9.14): **Follicular phase** (days 1--13): Stimulation of the HPG axis results in the release of **gonadotropin-releasing hormone (GnRH)** from the hypothalamus, stimulating the anterior pituitary gland to release small amounts of **follicle-stimulating hormone (FSH)** and **luteinizing hormone (LH)**. FSH and LH stimulate developing ovarian follicles to release estrogens. During the early follicular phase, low to moderate estrogen levels exert negative feedback on the HPG axis, and only one developing  Chapter 9: Reproductive System 334 follicle typically survives. The surviving (dominant) follicle secretes **inhibin**, which acts on the anterior pituitary to inhibit FSH, repressing maturation of additional follicles during the same ovarian cycle. Although estrogen initially inhibits the HPG axis, the emergence of a dominant follicle stimulates the secretion of high levels of estrogen, exerting a *stimulatory* effect on the HPG axis during the late follicular phase. This effect results in a surge of LH and, to a lesser extent, FSH. **Ovulation** (day 14): Soon after the LH surge, the mature ovarian follicle ruptures, and a secondary oocyte is released into the abdominal cavity, entering a nearby uterine tube. **Luteal phase** (days 15--28): LH stimulates the conversion of the ruptured follicle into a structure known as the **corpus luteum**, which secretes high levels of progesterone and estrogens, exerting negative feedback on the HPG axis. Lower FSH and LH levels during the luteal phase prevent maturation of additional follicles. If fertilization does not occur, the corpus luteum degenerates, causing a sharp decline in progesterone and estrogens. This decline relieves the inhibition of FSH and LH, allowing the next ovarian cycle to begin. If the secondary oocyte is fertilized following ovulation, the corpus luteum persists and continues to secrete progesterone and estrogens. The continued presence of progesterone and estrogens keeps FSH and LH levels low, promoting the pregnancy and preventing a new ovarian cycle from being initiated during pregnancy. **Figure 9.14** The events of the ovarian and uterine cycles. A diagram of a diagram of the ovulation cycle Description automatically generated Chapter 9: Reproductive System 335 The uterine (menstrual) cycle occurs concurrently with the ovarian cycle and is also divided into three phases (Figure 9.14): **Menstrual phase** (days 1--4): If fertilization of a secondary oocyte does not occur in the prior uterine cycle, **menstruation** begins. During **menses**, the uterus sheds the majority of its endometrial (ie, inner) layer. The detached tissue and blood pass out of the body through the vagina. Toward the end of this phase, ovarian follicles are stimulated to grow and produce estrogens, resulting in the cessation of blood flow. **Proliferative phase** (days 5--14): The endometrial layer proliferates, doubling in thickness. Endometrial glands develop and vascularization of the endometrium is increased to prepare for embryo implantation. **Secretory phase** (days 15--28): Increased progesterone and estrogen secretion from the corpus luteum triggers the further thickening and development of the endometrium. These changes result in secretion of nutrients from endometrial glands and the creation of an environment that can sustain a developing embryo in the event of fertilization and implantation